1. Field of the Invention
The present invention relates to an image decoder and an image decoding method for obtaining image data whose size is reduced, from image data which has been compression-coded.
2. Description of the Related Art
When reproducing image data which has been compressed in a compression method which supports interlaced scanning such as MPEG 2 (Moving Pictures Experts Group phase 2), etc., there are some cases where such image data is displayed on a display device having resolution different from the size of the image data. Those cases include video data for a HDTV (High Definition Television) displayed on an ordinary television monitor, and a PC (personal computer) monitor.
Generally, in those cases, the image data is completely decoded, and then, the data size is reduced at the time the image data is displayed. When reducing the data size, details (high frequency components) of the image are lost. Therefore, by previously cutting the high frequency components and then decoding the image data together with reducing the data size, reproducing performance is improved. In a case where image data is decoded by software on a PC, even if decoding and reproducing are performed on a low-performance CPU, load imposed on the CPU while decoding can be reduced if decoding is performed while reducing the resolution.
“Scalable decoder without low-frequency drift” by Iwahashi, Kanbayashi, and Takaie (Technical Report of IEICE, DSP94-108, 1995.1) proposes a decoder which reduces the resolution during decoding, and thus outputs an image whose size is reduced, as described above. FIG. 6 is a block diagram showing the structure of the decoder of this proposal. When decoding compressed image data, the 4×4 IDCT (Inverse Discrete Cosine Transform) process unit 623 of this decoder reduces the resolution by performing 4×4 IDCT using only low frequency components among DCT blocks. The motion compensation process unit 625 reduces the values of a decoded motion vector to half, and performs motion compensation with ¼ pixel precision.
However, according to this decoding method, field information of each line of the image is lost by reducing the size in a vertical direction. Thus, there arises a problem that field prediction of the motion compensation employed in MPEG 2, etc. cannot be performed properly.
To overcome this problem, Unexamined Japanese Patent Application KOKAI Publication No. 2000-59793 discloses a method of switching IDCT methods between a field DCT mode and a frame DCT mode. According to this method, when the mode is switched to the frame DCT mode which applies DCT to the top field and to the bottom field together, image data which is obtained by performing IDCT is once divided into two fields, and field DCT is applied to the respective fields. And by performing IDCT using only low frequency components, size reduction is performed while keeping field information.
However, this method includes many calculations, thus is not suitable for processes that require high process performance, such as a decoding process performed by software on a PC. And this method is completely unsuitable in a case where display resolution is reduced with a view to improving reproducing performance.
Unexamined Japanese Patent Application KOKAI Publication No. 2000-175195 discloses a technique by which, when the mode is set to the frame DCT mode, IDCT is applied to all coefficients of DCT blocks, and DCT is applied to respective two pixel blocks which have been separated. However, this technique cannot solve the problem of increase in calculations, either. Further, as another example of a known art, Unexamined Japanese Patent Application KOKAI Publication No. 2000-244917 discloses a technique for providing an image decoder and an image decoding method which simplify the decoding process using a simple structure at the time of motion compensation. Unexamined Japanese Patent Application KOKAI Publication No. 2000-350207 discloses a technique for applying a transformation kernel including less calculations to an up sampling and down sampling procedures by employing generalized orthogonal transformation. Further, Unexamined Japanese Patent Application KOKAI Publication No. H10-191338 discloses a technique relating to a single coding apparatus capable of compressing an image effectively regardless of whether the image is a natural image or an artificial image.